The application of conductive material to the inner surfaces of the through holes, referred to as through hole printing, is presently performed using manual print systems. An operator hand loads a substrate onto a support referred to in the art as a print nest. The operator locates the substrate on the print nest so that the through holes in the substrate are aligned with corresponding holes in the print nest. The operator then activates a hold down vacuum, which is applied through holes in the nest located adjacent to the perimeter of the substrate, to clamp the substrate to the nest.
The print nest is thereafter pneumatically moved into a printing position under a printing screen. The printing operation is then performed whereby the desired pattern of conductive paths, including the conductive material for the through holes, is printed on the substrate, and the nest is moved back to the load/unload position. The operator then activates a second vacuum that is applied to the corresponding holes in the nest and serves to pull the conductive material through the through holes in the substrate and coat the inner walls of the through holes. U.S. Pat. No. 4,710,395 issued to Young et al. on Dec. 1, 1987 discloses a method and apparatus that can be used to increase the uniformity of this coating operation.
This completes the printing operation, and the operator deactivates both vacuums and removes the substrate from the print nest. Before loading the next substrate, the operator must wipe away excess conductive material that has been pulled through the through holes of the previous substrate onto the surface of the nest. Otherwise, this excess conductive material will smear on the bottom surface of the next substrate and cause electrical shorts between conductive paths on the lower surface.
It is clear that the foregoing process is both labor intensive and time consuming and thus makes the product produced by this process expensive. Furthermore, this process does not lend itself to automation in view of the need to wipe away the excess conductive material from the print nest each time a through hole printing operation is performed.